1. Field of the Invention
The present invention relates to a resin particle liquid dispersion for an electrostatic image developing toner excellent in various properties such as color formability and OHP transparency and suitably usable for forming an image by an electrophotographic process, an electrostatic recording process or the like, a toner using the resin particle liquid dispersion, production processes thereof, a developer using the toner and an image forming method.
2. Description of the Related Art
With rapid spread of digitization technology, high image quality is currently demanded for the output such as print and copy by users at the home or office or in the publishing field. In order to meet this requirement for high image quality, the toner for use particularly in electrophotography is reduced in the particle diameter with an attempt to enhance the resolution, and the particle diameter is at present reduced even to the region of 5 μm. In this case, in view of production energy and cost, reduction in the toner particle diameter can be hardly attained by a kneading-pulverization method conventionally employed for the production of toner, and the trend is now shifting to a technique of preparing a resin particle liquid dispersion by a so-called chemical production process in an aqueous medium, such as suspension-polymerization, dissolution-suspension or emulsion polymerization-aggregation, and granulating the resin particle liquid dispersion to produce a toner.
In general, control of particle diameter and particle diameter distribution of toner particles is indispensable for a toner so as to obtain a high-quality image. A toner produced particularly by a kneading-pulverization or suspension-polymerization method having a tendency of bringing about a broad particle diameter distribution passes through a classification step to obtain toner particles having a desired particle diameter and therefore, the yield is liable to decrease.
With respect to the method for polymerizing a resin in an aqueous medium, a suspension-polymerization method, a dissolution-suspension method and an emulsion polymerization-aggregation method are generally employed at present. As for the emulsion polymerization, the polymerization of monomers by using a surfactant in an amount less than the critical micelle concentration (CMC) and a co-surfactant in combination in the presence of a polymerization initiator for the monomer emulsion is particularly known as so-called “miniemulsion polymerization” which is described, for example, in P. L. Tang, E. D. Sudol, C. A. Silbi and M. S. El-Aasser, J. Appl. Polym. Sci., Vol. 43, page 1059 (1991).
The conventional emulsion polymerization method is a method of polymerizing an aqueous emulsion of monomer particles having a particle diameter of about several μm by using a water-soluble polymerization initiator in the presence of a surfactant not less than the critical micelle concentration (CMC). In the conventional method, the polymerization is initiated in a surfactant micelle and a monomer is diffused and supplied from the monomer particle to grow and form polymer particles.
On the other hand, the miniemulsion polymerization method is advantageous in that since a monomer is polymerized in the monomer particle, migration (e.g., diffusion) of a substance is not involved, and polymer particles uniform in the shape and properties can be formed. That is, in the miniemulsion polymerization method, the diameter of polymer particles can be controlled by controlling the liquid droplet size of a monomer liquid dispersion.
Examples of the invention relating to a toner produced by using such a miniemulsion polymerization method include the followings. For example, JP-A-2001-290308 discloses a production process of an electrostatic image developing toner, comprising aggregating colorant-containing polymer particles obtained by miniemulsion polymerization of a monomer and a colorant. JP-A-2002-49180 discloses a toner obtained by salting-out/fusing resin particles produced through multi-step polymerization, where the resin particle except for the outermost layer contains a resin particle produced by using miniemulsion polymerization. These studies all are made on the toner production process involving a radical polymerization reaction by miniemulsion polymerization.
In Mattheineu Barrere and Kaharins Landfester, Polymer., Vol. 44, page 2833 (2003), the results when a polyester is polycondensed from a dodecane diacid and a dodecane diol or the like in water are described. However, in this and other research papers, there is not found a case of studying on physical properties of a polycondensed resin particle obtained by a miniemulsion polymerization method or on a technique for obtaining a polycondensable polymer having specific physical properties. Needless to say, a case of studying on the application of a miniemulsion polymerization method comprising a polycondensation reaction to the production of a toner is not found.
Also, with the increasing demand to save energy necessary for the image formation, the toner fixing temperature must be more lowered so as to realize power saving at the fixing step. When the toner fixing temperature is lowered, in addition to power saving, the waiting time until the fixing member surface reaches the fixing possible temperature after turning on the power source, so-called warm-up time, can be shortened and the life of the fixing member can be prolonged.
For lowering the toner fixing temperature, a method of decreasing the glass transition point of a toner particle is generally employed. However, if the glass transition temperature is excessively lowered, aggregation (blocking) of powder particles readily occurs or preservability of the toner on the fixed image is lost (generally called “document offset”), and lowering of the toner fixing temperature and preservability of the toner can be hardly attained at the same time. In order to attain both low-temperature fixing and toner preservability, a so-called sharp-melt property of causing abrupt decrease in the viscosity of the toner in a high-temperature region must be imparted while keeping the glass transition point of the toner at a higher temperature.
However, the resin used for the toner usually has a certain width in the glass transition point, molecular weight or the like and therefore, the composition and molecular weight of the resin must be made extremely uniform so as to obtain a sharp-melt property. For obtaining such a resin, it is necessary to use a special production process or adjust the molecular weight of the resin by subjecting the resin to a treatment such as chromatography. In this case, the cost inevitably rises or an unnecessary resin (waste) is yielded at the production of a highly uniform resin and this is not preferred also from the aspect of environmental protection awareness in recent years.
Accordingly, a toner for electrophotography having a low fixing temperature and a sharp-melt property and causing no offset even in a high-temperature region is keenly demanded.
In general, as means for satisfying all of blocking inhibition, image preservability and low-temperature fixing, a method of using a crystalline resin as the binder resin has long been known (see, for example, JP-B-4-24702). The crystalline resin has a melting point to afford great decrease in the viscosity at a specific temperature, and the temperature difference between the start of thermal activity of a resin molecule and the fixing possible region can be made small, so that an excellent low-temperature fixing property can be imparted. On the other hand, in the case of a non-crystalline resin, the viscosity gradually decreases after the resin molecule starts its thermal activity at the glass transition point, and the temperature difference until the fixing possible region is large, as result, a low-temperature fixing property cannot be ensured. In the crystalline resin, the viscosity tends to continuously decrease even at a temperature higher than the melting point, and the melted toner permeates into paper to provide an effect of preventing generation of offset, but excessive permeation of the melted toner into paper disadvantageously occurs to cause a problem that an image with uniform and high density cannot be obtained. Furthermore, the crystalline resin has insufficient hardness at normal temperature and, for example, this may allow for deformation of the toner particle on mixing with a carrier in a developing machine or deformation of the toner particle by the shear force imposed from a cleaning blade, giving rise to deleterious change of electrification cleaning failure or insufficient strength of the produced image.
In order to solve these problems, many techniques of using a crystalline polymer as the binder resin in combination with a non-crystalline polymer have been proposed. For example, JP-A-2001-42564 proposes a technique of fusing resin particles containing a crystalline substance and an amorphous polymer, in an aqueous medium. However, in this invention, a crystalline polyester resin is produced by a conventional method. Generally, in the case where a polyester resin produced through polycondensation at a high temperature of 150° C. or more is used for the chemical process toner, there is a problem that a huge energy is necessary to disperse and emulsify the once bulk-polymerized resin to a toner diameter, the use and recovery of an organic solvent requires a large-scale facility investment, or the obtained resin particle liquid dispersion suffers from a board resin particle diameter distribution. Furthermore, in the above-described method, it is difficult to effect aggregation and coalescence of the toner by mixing a latex containing a crystalline substance with a latex containing an amorphous substance. This is ascribable to the difference between the surface electric charge of the crystalline polyester latex and the surface electric charge of the vinyl-based latex.
Similarly, JP-A-2004-191927 discloses an electrostatic image developing toner comprising a crystalline polyester as the binder resin and an amorphous polymer, the toner surface being covered with a surface layer mainly comprising the amorphous polymer, wherein the crystalline polyester content is from 30 to 80 wt %, the proportion of the crystalline polyester contained in the outermost surface of the electrostatic image developing toner is 15 atomic % or less, and the average thickness of the surface layer is from 0.01 to 0.5 μm. However, this toner is produced by mixing an amorphous polymer liquid dispersion and a crystalline polyester liquid dispersion at the aggregation step, and uneven distribution of constituent components is liable to occur during aggregation. Furthermore, the method of producing the crystalline polyester liquid dispersion has a problem that a high energy is necessary as described above or the production process is long and complicated.
Also, many techniques of using a polymer obtained by chemically bonding a crystalline polymer and a non-crystalline polymer have been invented. Examples thereof include JP-A-2004-233983. This invention is a toner comprising a resin formed by reacting an epoxy group of an epoxy group-containing vinyl resin with a carboxyl group of a resin containing a polyester unit and a carboxyl group, wherein the toner has a specific viscoelasticity. However, since a reaction process at a high temperature is necessary for obtaining resin particles by this production process as in conventional techniques and the toner is produced by a melting-kneading production process, the energy consumption is large and the obtained toner has a hardly alignable shape and a broad particle diameter distribution. JP-A-2001-117268 also discloses a toner comprising a crystalline polyester having a polymerization crosslinked structure introduced by a radical reaction of unsaturated bonds, but the toner produced by suspension polymerization has a broad particle diameter distribution and therefore, high image quality is hardly obtained. Furthermore, the polycondensation reaction is based on a conventional method and requires a high energy.
In this way, as for the toner using a crystalline resin, it is difficult to achieve low-energy production of a toner successfully controlled in the viscoelasticity, electrostatic property, toner particle diameter and particle diameter distribution. Also, in the mixing of a crystalline resin and a non-crystalline resin, various contradictory properties are required to satisfy at the same time, and a toner satisfied in these and other toner properties such as powder fluidity and coloring property is not provided at present.